What are containers

A standard unit of software that packages code and all its dependencies so the application runs quickly and reliably from one computing environment to another. A single container image is executable package of software that includes everything needed to run an application – code, runtime, system tools, system libraries, configuration files, and settings, bundled into one package. By containerizing the application platform and its dependencies, differences in OS distributions and underlying infrastructure are abstracted away.

Container images become containers at runtime.  Containers are a solution to the problem of how to get software to run reliably when moved from one computing environment to another. This could be from a developer’s laptop to a test environment, from a staging environment into production, and perhaps from a physical machine in a data center to a virtual machine in a private or public cloud. 

Containers vs. virtualization

  • Containers are different than virtualization technology, where the package that can be passed around is a virtual machine, and it includes an entire operating system as well as the application. A physical server running three virtual machines would have a hypervisor and three separate operating systems running on top of it. By contrast, a server running three containerized applications with Docker runs a single operating system, and each container shares the operating system kernel with the other containers. Shared parts of the operating system are read only, while each container has its own mount (i.e., a way to access the container) for writing. So, containers are more lightweight and use fewer resources than virtual machines.

What other benefits do containers offer?

  • A container may be only tens of megabytes in size, whereas a virtual machine with its own entire operating system may be several gigabytes in size. Because of this, a single server can host far more containers than virtual machines.
  • Virtual machines may take several minutes to boot up their operating systems and begin running the applications they host, while containerized applications can be started almost instantly. That means containers can be instantiated in a “just in time” fashion when they are needed and can disappear when they are no longer required, freeing up resources on their hosts.
  • Containerization allows for greater modularity. Rather than run an entire complex application inside a single container, the application can be split in to modules (such as the database, the application front end, and so on). This is the so-called microservices approach.  Applications built in this way are easier to manage because each module is relatively simple, and changes can be made to modules without having to rebuild the entire application. Because containers are so lightweight, individual modules (or microservices) can be instantiated only when they are needed and are available almost immediately.

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